The ditching was really well done, almost looks like it aqua planed for a short while before biteing in. The prop can be seen slowing to a stop in the last few seconds, so pure luck it went horizontal.
As for cause, they said they had full fuel at the start. Many things can go wrong with the fuel system, which seems the likely area to focus on as the prop was rotating until low speed suggesting oil or a catastrophic mechanical failure unlikely.